A transmitter of a communication device, such as a cellular phone and a wireless LAN, is required to operate with low power consumption while securing the accuracy of transmitting signals irrespective of its output power. In particular, the power consumption of a power amplifier, which is arranged in the last stage of a communication device and which occupies 50% or more in the entire power consumption of a communication device, is required to operate at high power efficiency.
Recently, a switching amplifier has attracted attention as a power amplifier which is expected to demonstrate high power efficiency. A switching amplifier is assumed to receive a pulse waveform signal as its input signal, thus achieving power amplification with maintaining its waveform. A pulse waveform signal amplified by a switching amplifier is adequately suppressed in frequency components, other than a desired frequency component, with a filter element and subsequently emitted into the air via an antenna.
FIG. 19 is a circuit diagram showing a D-class amplifier 1 representative of a switching amplifier. The D-class amplifier 1 includes two switch elements 3a, 3b which are connected in series between a power source 2 and a ground GND. Complementary pulse signals S1, S2, serving as open/close control signals, are applied to the two switch elements 3a, 3b, thus solely turning on either the switch elements 3a, 3b. When the switch element 3a proximate to the power source 2 is turned on while the switch element 3b proximate to the ground GND is turned off, the D-class amplifier 1 outputs a voltage identical to a source voltage. In contrast, when the switch element 3a is turned off while the switch element 3b is turned on, the D-class amplifier 1 outputs a voltage identical to a ground potential.
Ideally, the D-class amplifier 1 causes zero power loss because it does not need a bias current. In this connection, the switch elements 3a, 3b may comprise field-effect transistors or bipolar transistors.
FIG. 20 is a block diagram showing the constitution of a wireless transmitter 5 using the D-class amplifier 1. This constitution is disclosed in NPL 1. In FIG. 20, the constituent parts identical to those shown in FIG. 19 are denoted using the same reference signs. The wireless transmitter 5 includes an RF signal generation circuit 6, a driver amplifier 7, and the D-class amplifier 1. In the case of the W-CDMA (Wideband Code Division Multiple Access) system, for example, a digital baseband 8 is able to generate a multi-bit wireless signal configured of ten bits or more.
The D-class amplifier 1 receives a pulse waveform signal as its input signal. An output signal of the digital baseband 8 needs to be converted into 1-bit information in advance because a pulse waveform signal is able to solely transmit one bit per one pulse. Sigma-delta modulators 9a, 9b are employed as a circuit configuration achieving one-bit information conversion in order to maintain a good noise characteristic in frequencies proximate to a frequency band of a desired wave. With this circuit configuration, it is possible to convert a wireless transmission signal into a pulse waveform signal while maintaining a good noise characteristic, thus inputting a pulse waveform signal into the D-class amplifier 1.